Sending or receiving?

Track-backs to mu.nu have been shut down for everyone for some time due to highly aggressive spam activity. I believe it was the only way to stop the spam and actually keep the servers online.

Both. I tried to trackback a few other mu.nu blogs tonight with no luck. I have noticed the problem for awhile, it just took me awhile to ask. I figured it might fix itself.... Or Pixy would work his majic!

Yup, not going to happen to ANY mu.nu blog as of the last update. You can send trackbacks to non-mu.nus, but that's about it.

Incoming trackbacks have been down since the DDoS attack in June - not related, but once the attack started I just didn't have the time to keep trackbacks going (and I accidentally clobbered my spammer blacklist).

At last count, we were getting 7 or 8 million trackbacks a month, of which less than 0.01% were valid.

Now, outgoing trackbacks are working in general, but the blog you are pinging is quite likely also having problems.